In line with recent rapid emergence of energy reduction and environmentally-friendly green growth industries as a new issue, research into hybrid vehicles using internal combustion engines along with motors or fuel cell vehicles utilizing hydrogen, an environmentally-friendly energy source, as alternative energy, to generate electricity and drive motors have been actively conducted in automotive industries. Since these environmentally-friendly vehicles are driven by electric energy, permanent magnet type motors and generators are inevitably employed, and technical demands for rare earth permanent magnets exhibiting better hard magnetic performance tend to be increased in order to further increase energy efficiency in terms of magnetic materials.
Also, in other aspects for improving fuel economy of automobiles, weight reductions and miniaturization of automotive parts must be realized, and for example, with respect to motors, it is essential to replace permanent magnet materials with rare earth magnets exhibiting magnetic properties better than those of typically used ferrites along with design changes of the motors, in order to realize the weight reductions and miniaturization.
Residual magnetic flux density of a permanent magnet is theoretically determined by conditions, such as saturation magnetic flux density of a main phase constituting a material thereof, a degree of anisotropy of grains, and density of the magnet. Since the magnet may generate stronger magnetic force toward the outside as the residual magnetic flux density increases, efficiency and performance of devices may be improved in various applications. Also, since coercive force among magnetic properties of the permanent magnet acts to maintain inherent performance of the permanent magnet corresponding to environments demagnetizing the magnet, such as heat, a magnetic field in an opposite direction, and mechanical shock, environmental resistance is good when the coercive force is excellent, and thus, the magnet may not only be used in devices for high-temperature application and high-power devices, but the thickness thereof may also be decreased. Therefore, the weight thereof may be reduced to increase its economic value. R—Fe—B-based rare earth magnets have been known as permanent magnetic materials exhibiting excellent magnetic performance.
However, since expensive rare earth elements are used as main raw materials for rare earth permanent magnets, manufacturing costs thereof are higher than those of ferrite magnets, and thus, the price of a motor may not only increase as the rare earth permanent magnets are employed, but there may also be limiting factors of resources in which reserves of the rare earth elements are not abundant in comparison to those of other metals. Therefore, there is a need to invent a method of manufacturing low-cost magnets by recycling wasted rare earth magnets in order to expand applications of rare earth magnets and address limitations in supply and demand.
Meanwhile, R—Fe—B-based rare earth magnets are manufactured in the form of a sintered magnet or bonded magnet by using R—Fe—B alloys as a starting raw material. Rare earth sintered magnets are manufactured by general powder metallurgical processes and machining, and scraps in an amount ranging from about 30% to 40% are generated during the manufacturing processes (annual amount of scrap generated based on 2008: 58,000 ton/year×0.35=20,300 ton/year). However, since almost of these expansive rare earth magnet scraps may not be reused and a process of extracting rare earth elements only by refining may be performed, additional processing costs for recycling may be required.
Therefore, attempts to recycle low-cost stating materials, such as process scraps generated during the foregoing manufacturing processes of rare earth sintered magnets and rare earth sintered magnet products recovered from defective products or wasted products, as powders for low-cost rare earth bonded magnets have been underway. According to typical techniques used in the art, these rare earth scraps are ground and prepared as powder having a diameter ranging from 50 μm to 500 μm and the powder is then mixed with a thermosetting resin, such as epoxy, to manufacture rare earth bonded magnets through a forming process and a curing process at a temperature ranging from 100° C. to 150° C.
However, when rare earth powder and bonded magnets are prepared by the foregoing processes, magnetic defects, such as oxidation or mechanical residual stress during the grinding process, may be generated, and as a result, coercive force may be decreased with inversely proportional to a particle diameter of the powder. In particular, limitations in quality may occur, in which characteristics of the magnets may become unstable as effects of magnetic defects on the surfaces thereof may be further increased when the curing process at a temperature ranging from 100° C. to 150° C. is performed.
The present invention uses rare earth sintered magnet scraps as a staring raw material in order to significantly reduce manufacturing costs in manufacturing R—Fe—B-based powders for bonded magnets and aims at improving coercive force and thermal stability of the powders by using an improved Hydrogenation-Disproportionation-Desorption-Recombination (HDDR) method. Further, a method of preparing powder for a rare earth bonded magnet has been devised, in which R—Fe—B-based anisotropic powder having excellent magnetic performance, stable production, and uniform quality is prepared by using low-cost stating materials, such as process scraps generated during the foregoing manufacturing processes of rare earth sintered magnets and rare earth sintered magnet products recovered from defective products or wasted products, and the improved HDDR method, i.e., a method in which hydrogenation, disproportionation, and desorption processes are performed, and then the disproportionation and desorption processes are again repeatedly performed (Korea Patent Application No. 10-2009-0119785) and a recombination process is completed.